Infiltration and Injection of a Ca-Citrate-PO4 Solution to Form Apatite In Situ for Sr-90 Remediation at the Hanford 100N Area - An Update

Sr-90 present in groundwater and the vadose zone at the Hanford 100N area due to past waste disposal practices has reached the nearby Columbia River, as evidenced by Sr-90 concentrations in near river wells and aquifer tubes and near shore sediments. Sr-90 is currently being remediated by adsorption onto apatite (55 times stronger than Sr-90 adsorption to sediment), followed by incorporation of the Sr-90 into the apatite structure. If the Sr-90 can remain immobilized for 300 years (∼ten 29.1-yr half-lives of Sr-90 decay), it will have decayed below regulatory limits to Y-90 and to stable Zr-90. Apatite (Ca10(PO4)6(OH)2) is being precipitated in situ in saturated zone sediments by injection of a aqueous solution of Ca-citrate and Na-phosphate through a series of 16 wells. For the treatability study, field scale demonstration of the technology was implemented through injection of a low-concentration, apatite-forming solution, followed by high concentration solution injections as required to emplace sufficient treatment capacity to meet treatability test objectives. Analysis of field cores collected after the low concentration injections indicates that targeted apatite contents were achieved and that ∼25% of the Sr-90 associated with the sediment was incorporated in the apatite structure. Aqueous Sr-90 monitoring in four compliance monitoring wells over a year following the high concentration injections indicates 84% to 95% decrease in Sr-90 concentrations (relative to the low and high end of the baseline range, respectively). Cores are currently being analyzed to confirm the apatite mass and Sr-90 substitution in apatite after these high concentration injections.